Bullet impact force,velocity predicted by ballistic gelatin

Aim: To understand the physics(equations) behind this problem.
On the channel 'Discovery Science',a the show-presenter was testing guns.In one of the tests,he fires the gun at a certain distance from a 'BALLISTIC GELATIN'(just a jelly of high viscosity to simulate swine muscle tissue).
The he says,"By using the high-speed camera here,we can tell exactly how far the bullet has travelled through the jelly and hence determine the bullet's velocity and from the cross-section of the bubble inside the jelly,we can determine the impact force of the bullet".
Now,i can't seem to figure out how he can do that!

I can only start off with:
Kinetic energy=work done
,where 'work done=force*penetration depth' is the only thing i can think of.

Thanks!

**I assume there's no other accurate way of calculating the bullet speed,since its almost impossible to measure the velocity of gun recoil to good accuracy*(cons.of momentum)**

I think they're skipping over a lot of explanation in the show you mention, it seems to me there is no great way to know anything quantitative about the bullet's velocity or "impact force" using just ballistics gel. My guess is they might have some general empirical data from known bullets hitting the gel at known speeds, and they can infer some properties based on that.

Too right. The effectiveness of a round is usually determined by using the mass of the bullet and the speed. There are exceptions to that based upon things like armour-piercing capabilities, explosive warheads, etc., but it's normally pretty accurate.